Well as most of you probably noticed from the virst IV trailer where Niko is up on the wall there is the leader of the Diabo's form III and I found the wall its by Cerveza Heights in a corner a blocksouth of East island City theres a bunch of graffiti murals and theres more characters, 8-Balla Yardie I think and a Yakuza boss.

Edit * I also looked further and there's that one girl from Vice City with the black hairand pink bathing suit, I think thats Mercedes and Misty again from III, and CJ from San Andreasbeside two purple lettrs stating " M3 " They also have a helicopter from Vice City which is onthe cover of the game and the motorcycle.